Externalism is widely endorsed within contemporary philosophy of mind and language. Despite this, it is far from clear how the externalist thesis should be construed and, indeed, why we should accept it. In this entry I distinguish and examine three central types of externalism: what I call foundational externalism, externalist semantics, and psychological externalism. I suggest that the most plausible version of externalism is not in fact a very radical thesis and does not have any terribly interesting implications for philosophy of mind, whereas the more radical and interesting versions of externalism are quite difficult to support